Embodiments of the present invention relate to single-step processes for synthesizing polyimide oligomers having high thermal and oxidative stability and improved mechanical properties.
It is well known in the art that polyimides exhibit properties desirable for high-temperature applications such as those demanded in the aerospace industry. Existing materials are primarily based on Polymerization of Monomeric Reactants (PMR) chemistry and are, with a few exceptions, only suitable for prepreg. Additionally, these materials are plagued with various deficiencies. For example, PMR-15, which is described in U.S. Pat. No. 3,745,149, contains a known carcinogen 4,4-methylene dianiline (MDA), is prone to microcracking, difficult to handle during processing and has a short shelf life. Many PMR materials cannot be used for thick composites due to unwanted reaction byproducts.
Historically, the utilization of polyimides in high-temperature composite applications has been limited due to their processing cost and difficulties. Problems associated with processing polyimides for such applications include poor solubility in many solvents, evolution of volatiles during imidization and the high-temperatures necessary for processing.
A fundamental problem withal of the PMR-based chemistries (and many others as well) is that multiple reactions take place during the curing and crosslinking of these resins. Having multiple reactions taking place means that reactions other than those principally desired may take place. Furthermore, reactions at temperatures below the desired processing and cure temperatures can increase resin viscosity unacceptably. Accordingly, it is desirable to have resin systems for polymers that only have one reaction mechanism during curing. Moreover, it is highly desirable to have the single reaction mechanism occur between endcaps that have difunctionality.